I know you're talking about features more so than specific models, but I was tired of my iPhone 3G after using it for a couple of years. It was sufficient to me

I know you're talking about features more so than specific models, but I was tired of my iPhone 3G after using it for a couple of years. It was sufficient to me feature-wise, but the sluggish response time on every screen was just frustrating. However, after upgrading, iPhone 4 speed and retina display have been game-changing to me as a user.